1. Field of the Invention.
This invention relates in general to data storage devices, and more particularly to method and apparatus for providing amplitude instability data recovery for AMR/GMR heads.
2. Description of Related Art.
In a disk drive the MR head is mounted on a slider which is connected to a suspension arm, the suspension arm urging the slider toward a magnetic storage disk. When the disk is rotated the slider flies above the surface of the disk on a cushion of air which is generated by the rotating disk. The MR head then plays back recorded magnetic signals (bits) which are arranged in circular tracks on the disk.
The MR sensor is a small stripe of conductive ferromagnetic material, such as Permalloy (NiFe), which changes resistance in response to a magnetic field such as magnetic flux incursions (bits) from a magnetic storage disk. The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flowing through the read element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensed current or voltage. Conventional MR sensors based on the AMR effect thus provide an essentially analog signal output, wherein the resistance and hence signal output is directly related to the strength of the magnetic field being sensed.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. This GMR effect has been found in a variety of systems, such as Fe/Cr, Co/Cu, or Co/Ru multilayers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers. This GMR effect has also been observed for these types of multilayer structures, but wherein the ferromagnetic layers have a single crystalline structure and thus exhibit uniaxial magnetic anisotropy, as described in U.S. Pat. No. 5,134,533 and by K. Inomata, et al., J. Appl. Phys. 74 (6), Sept. 15, 1993. The physical origin of the GMR effect is that the application of an external magnetic field causes a reorientation of all of the magnetic moments of the ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus a change in the electrical resistance of the multilayered structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes. MR sensors based on the GMR effect also provide an essentially analog signal output.
In high density disk drives bits are closely spaced linearly about each circular track. In order for the MR head to playback the closely spaced bits the MR head has to have high resolution. This is accomplished by close spacing between the first and second shield layers, caused by thin first and second gap layers, so that the MR sensor is magnetically shielded from upstream and downstream bits with respect to the bit being read.
An MR head is typically combined with an inductive write head to form a piggyback MR head or a merged MR head. In either head the write head includes first and second pole pieces which have a gap at a head surface and are magnetically connected at a back gap. The difference between a piggyback MR head and a merged MR head is that the merged MR head employs the second shield layer of the read head as the first pole piece of the write head. A conductive coil induces magnetic flux into the pole pieces, the flux flinging across the gap and recording signals on a rotating disk. The write signals written by the write head are large magnetic fields compared to the read signals shielded by the first and second shield layers. Thus, during the write operation a large magnetic field is applied to one or more of the shield layers causing a dramatic rotation of the magnetic moment of the shield layer.
Magnetic recording data storage technologies, particularly magnetic disk drive technologies, have undergone enormous increases in stored data per unit area of media (areal data density). This has occurred primarily by reducing the size of the magnetic bit through a reduction in the size of the read and write heads and a reduction in the head-disk spacing.
However, it has been found that some AMR and/or GMR heads exhibit severe amplitude instability such that data cannot be properly read from the disk. In this instance, an error is detected which in turn triggers some corrective action. An error detected while the data is being read form the disk is commonly referred to as a read error, a soft read error is an error that is possible to correct. Many times the correction of the read error is handled without interrupting the computer system which is beyond the rotating disk storage device. The soft read error would also be corrected before the user becomes aware of it.
A multistep procedure referred to as a data recovery procedure is (DRP) attempted to recover data when the storage device encounters a soft error. When the steps in the data recovery procedure are unable to correct a read error, then the read error is referred to as a hard error. Hard errors mean that data have been lost. Once data are read with a high error rate or lost from a particular portion of a disk, such as a sector, the area is reallocated to another spare portion on the disk drive. During the reallocation, some errors may be recovered.
For example, one problem encountered with MR sensors is Barkhausen noise caused by the irreversible motion of magnetic domains in the presence of an applied filed. It is know that Barkhausen noise is eliminated by creation of a single magnetic domain in the sense current region of the MR element. However, multiple magnetic domains may be formed during fabrication of the MR element. Further, as the dimensions of MR and GMR heads decrease, the MR and GMR heads are increasingly susceptible to low level electrical stress (ES) events that can cause the amplitude of the heads to become unstable and create a high number of soft or hard error events.
For example, the head amplitude can become suddenly as low as half of the normal value, and then become normal again, and after an unpredictable period become abnormal once again during write/read operations.
It can be seen that there is a need for a method and apparatus for providing amplitude instability data recovery for AMR/GMR heads.